As a packet service and an intelligent terminal rapidly develop, a high-speed service with a large data amount has an increasing requirement for a frequency spectrum. A millimeter wave is to become a potential target frequency spectrum for developing 5G (5th Generation) communications and 3GPP (3rd generation partnership project) Long Term Evolution Advanced (LTE-A) in the future. A centimeter wave frequency band generally refers to a frequency spectrum ranging from 3 GHz to 30 GHz, and a millimeter wave frequency band generally refers to a frequency spectrum ranging from 30 GHz to 300 GHz.
In the prior art, cellular communications such as LTE usually uses a frequency band of approximately 2 GHz or lower. An LTE-A small cell enhancement standardization project is studying and using a frequency band of 3.5 GHz. In the IEEE (Institute of Electrical and Electronics Engineers) 802.11ad standard, a frequency band of 60 GHz is used for a wireless local area network (WLAN), and is usually used for indoor communication at a short distance of approximately 10 meters. In the prior art, a frequency band of 6 GHz or higher has not been used in the cellular communications. A main challenge of using a millimeter-wave high frequency band in the cellular communications lies in that this wave band faces relatively large free-space attenuation; in addition, extremely severe attenuation is caused by absorption and scattering from air, rain, fog, buildings or other objects. A beamforming technology is considered as a potential technology that can compensate for a severe millimeter wave path loss. A massive multiple-input multiple-output (MIMO) system is considered as a potential direction for implementing the beamforming technology on a millimeter-wave frequency band.
The IEEE 802.11ad standard supports beamforming. A process of beam training between two nodes in communication is as follows: Anode 1 separately sends training beacons in a plurality of different directions by using a beam, and a node 2 receives the training beacons in a quasi-omni manner and identifies an optimal beam a; then the node 2 separately sends beacons in a plurality of different directions by using a beam, and the node 1 receives the beacons in a quasi-omni manner and identifies an optimal beam b; and the node 2 reports the optimal beam a to the node 1, and the node 1 reports the optimal beam b to the node 2, so as to find an optimal matched beam pair. Subsequently, data communication is performed in directions of the beam pair. However, on a low frequency band in the cellular communications, a public signal of a cell such as a synchronization channel or a broadcast channel is usually transmitted in an omni transmission manner rather than by using the beamforming technology.
In LTE-A carrier aggregation (CA), larger bandwidth can be obtained by aggregating a plurality of contiguous or non-contiguous component carriers (CC), so that a peak data rate and a system throughput are increased. CCs aggregated for UE are referred to as a serving cell. The serving cell includes one primary cell (PCell) on a low frequency band and at most four secondary cells (SCell) on a high frequency band. The primary cell is responsible for security of a non-access stratum (NAS), and the secondary cell mainly provides additional radio resources for data communication. CA supports handover of the PCell and adding, deleting, activation, deactivation, and other operations on the SCell.
A downlink synchronization channel of the SCell may be sent in a short distance point-to-point communication manner in 802.11ad, or sent through omni transmission in the cellular communications. In addition, in the short distance point-to-point communication manner in 802.11ad, a base station and UE need to separately send training beacons to each other in a plurality of different directions, so as to perform omni or quasi-omni beam training. A beam training process is relatively complex, and has a relatively large delay and relatively low system efficiency. Consequently, channel resources are wasted when the downlink synchronization channel is transmitted in the omni transmission manner. When the millimeter-wave high frequency band is applied to a cellular communications system, the base station may use the beamforming technology to extend public channel coverage. However, in the prior art, a method for performing downlink synchronization in an SCell cannot be provided when beamforming is directly applied to the cellular communications system.